What is a communication language?

A communication language, also called a preferred language, is the language a contact wants to be written to, stored as a value on their contact record. CRMs and signup forms record the same thing as a language preference. An email platform reads it at send time to choose which language version of a message the contact receives, and often to set the language of the pages behind the unsubscribe link.

How a communication language works

A communication language is ordinary contact data, written by a signup form, an import or an API call, and it usually holds a language code such as de or fr. It does nothing until a send reads it. In a multilingual campaign the platform compares each recipient's value with the languages of the template and delivers the matching version.

A contact without a usable value still receives the email. They get the template's primary language, the version that acts as the fallback for every recipient who cannot be matched. The quality of the routing therefore depends on how many contacts have a value and how consistently those values are written.

How to collect a contact's language

The most reliable value is one the contact supplied. A language selector on the signup form, defaulted to the language of the page, produces clean codes at no cost. Where a form cannot ask, the language of the page the person signed up on is the next best source, so a signup on /de/ is stored as de.

The browser's Accept-Language header, with values such as de-AT or en-GB, works as a last resort. Country is a weaker signal, because Switzerland, Belgium and Canada each have several languages and expats read their own. A name, a phone prefix or a billing address is weaker still. A wrongly guessed language reads worse to the recipient than the primary language would have.

Common problems with communication language data

• Several competing properties: a lang column from a CRM export, a locale from the application database and a language field from a newsletter form each cover part of the audience. The fix is to pick one, backfill it from the others and route on that one only.
• Free-text values: fields that accept typing collect German, Deutsch, n/a and typos next to proper codes. A review of the distinct values once a quarter shows which ones need cleaning.
• More than one language per contact: a value such as de,en is not a language, and routing treats it as unmatched.
• Legal text left untranslated: consent wording, the sender's identity and the unsubscribe mechanism have to be understandable to the recipient, and an English-only legal footer under a translated body undercuts that.

Communication language in Lettr

In Lettr, the communication language is a contact property with a designation. Ticking Use as communication language on a property under Audience → Properties makes it the language source for campaigns, and the property then shows a communication language badge. One property per team can hold the role, and ticking a second property moves it. Through the API the same switch is the purpose field, set to "communication_language" when a property is created or updated.

When no property is designated, Lettr looks for the language by property name in a fixed order: communication_language, communication_lang, language, lang, locale. The lookup runs per contact and the first name that holds a value wins, so a CSV imported with a lang column routes without any setup. Deleting the designated property shows no warning, and Lettr goes back to this name lookup.

The property has to be of type string. The switch is offered for every type without an error, but values in number, boolean, date or json properties are ignored, which sends every recipient the primary language, and a property's type cannot be changed later. A value made only of spaces counts as empty at send time, although the segment operator is empty does not catch it, so the count in the campaign's review panel is the one to trust. An API write that uses a property key the team has not defined is rejected with One or more property keys are not registered for your team.

The same value sets the language of the hosted preference center, the Manage email preferences page, and of the unsubscribe page, both available in 19 languages. They use the contact's value when it is one of the 19, then the team's Default communication language under Settings → Team, then English. The web version of a campaign is separate and always shows the language that was sent.

When no contact property holds a language, a campaign shows an amber warning in the Audience step and in the review panel, stating that every recipient will get the primary language, with a Set a language property button. The Contact Language page lists the full matching rules.